The oligo- or poly-merization of isocyanates, here referred to collectively as modification, has been known for a long time. If the modified polyisocyanates comprise free NCO groups, which may also have been temporarily deactivated with blocking agents, they are extraordinarily high-quality starting materials for the production of a large number of polyurethane plastics materials and coating compositions.
A number of industrial processes for modifying isocyanates have become established, wherein the isocyanate to be modified, which in most cases is a diisocyanate, is generally converted by addition of catalysts and then, when the desired degree of conversion of the isocyanate to be modified has been reached, the catalysts are rendered inactive (deactivated or separated off) by suitable measures and the resulting polyisocyanate is generally separated from the unconverted monomer. A summary of these processes of the art is to be found in H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
A special form of isocyanate modification, which yields products having a high content of iminooxadiazinedione groups (asymmetrical isocyanate trimers) in the products, in addition to the isocyanurate structures (symmetrical isocyanate trimers, frequently referred to hitherto only as “trimers” for the sake of simplicity) which have been known for a long time, is described inter alia in EP 962 455 A1, EP 962 454 A1, EP 896 009 A1, EP 798 299 A1, EP 447 074 A1, EP 379 914 A1, EP 339 396 A1, EP 315 692 A1, EP 295 926 A1 and EP 235 388 A1. (Hydrogen poly)fluorides inter alia have been found to be suitable catalysts therefor.
A disadvantage of the known processes of the art is that the iminooxadiazinedione content in the products is only about 50%, based on the sum of symmetrical (isocyanurate) and asymmetrical trimer (iminooxadiazinedione) and, in the case of higher monomer conversion, on the one hand, and with variations in the reaction temperature, on the other hand, that content decreases further to values below or above an optimum which is to be found in each case.
Although the iminooxadiazinedione content in the products can be influenced in the desired direction by increasing the “HF content” in the catalyst, that is to say by changing from simple fluorides (which generally do not have long-term stability and gradually change to the difluoride form even without the external addition of HF) to difluorides, trifluorides, etc., this procedure has disadvantages (higher HF content in the waste process gas, which must be neutralized in a complex procedure, higher corrosivity of the catalyst solutions, etc.) which do not outweigh the advantages.